Toluene nitration process



July 5, 1949. T. J. J. HOEK y TOLUENE NITRATION PROCESS Filed July 51, 1947 wwwa, Pa/f Patented July 5, 1949 TOLUENE NITRATION PROCESS Theodorus J. J. Hoek, Geleen, Netherlands, as-

signor to De Directie van de Staatsmijnen in Limburg, Heerlen, Netherlands Application July 31, 1947, Serial No. 765,111 In the Netherlands June 12, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires June 12, 1965 9 Claims.

This invention relates to toluene nitrati-on processes and, more particularly, it is concerned with -a method for the manufacture of trinitrotoluene by continuous operation as contrasted to batch nitration methods.

The manufacture of trinitrotoluene by a lcontinuous method has long been desired in the explosives field. However, it is understood that a satisfactory commercial method for continuous nitration of toluene to trinitrotoluene has never been developed. Thus, either the methods proposed heretofore have been associated with inherently dangerous steps, or have involved the use of an extremely large and impractical number of nitration steps, or have produced yields which are very low in comparison with the highly concentrated spent acids which are obtained.

A principle object of this invention is the provisio-n of a new method for the nitration of toluene to produce trinitrotoluene. A further object is a provision of such a procedure which is conducted in a continuous manner. Further objects include the provision of such a process (l) which will produce high yields of product in proportion to `the amount and concentrations of the nitration acids employed, (2) which will permit the nitration to be completed at a maximum rate, thus, reducing the number of nitration steps to a minimum, :and (3) which may be carried out withou-t recourse to the use of intricate or involved apparatus These objects are accomplished according to the present invention by effecting the nitration of Itoluene to trinitrotoluene in a continuous operation by means of mixed acid containing free S03 with the operation separated into a plurality of divisions. The hydrocarbon `and acid are reacted in continuous parallel flow in each separate division, and at the end of each division the nitro product obtained in that division is separated from the mixed nitration acid and is treated by stronger acid in the subsequent operation division. Fresh mixed acid is used in the highest division of the process and spent acid form one or more of the higher divisions, mixed with nitric acid, is used in each lower division of the process.

The success of the present invention is dependent to a large extent on the discovery that mixed acids containing a large quantity `of S03 may be employed to nitrate toluene without undue oxidation of the organic material at a reaction velocity which makes possible successful continuous operation. The success of the invention is also dependent upon the discovery that a successful continuous toluene nitration procedure could be developed if the operation is divided into a plurality of divisions so that the reactants, namely the mixed acid and organic matter, are caused to undergo parallel or co-current flow in each of the separate operation divisions, but are made to undergo counter-current flow between each of the separate steps. The separation of the continuous procedure into separate divisions makes possible the application of the conditions of temperature, acid 4concentration and the like which have been found necessary for the successful operation of the process. Furthermore, the operation of the process in plurality of divisions makes possible the economical and ecient use of the nitration acids. Thus, the spent acid from one step may be used, preferably with the addition of a controlled amount of concentrated nitric acid, as the mixed acid for the next lower step so that the process can be conducted without addition of water to the acid during any of the operation.

It has been found that the separation of the nitration process into three separate divisions is desirable. In the rst division toluene is nitrated to mononitrotoluene, in the second division the mononitrotoluene thus produced is formed into a mixture of monoand dinitrotoluene and, finally, in the third division, this mixture of monoand dinitrotoluene is formed into the final product trinitrotoluene.

The course of the process as a whole is diagrammatically illustrated in the attached drawing which constitutes a flow diagram of the operation. In the drawing, I and 2 are nitration vessels in which the formation of mononitrotoluene takes place. The reaction mixture, after having passed I and 2, enters into the separator 3, from which mononitrotoluene runs over to the nitration vessel 4 of the secon-d division. The reaction mixture formed here is next separated in the separator 5. The upper layer, being a mixture of monoand dinitrotoluene is led to the nitration vessels B, I and 8 to be cooled afterwards in the crystallizers 9 and III. The removal of the crystallized product takes place in the centrifuge II; the remaining acid is returned to lower operation divisions through `the conduit I2, save the part that is let off through conduit I3. The rest is led partly through I4 to 4, partly through I5 to I. In 4 the nitrating acid mixture is formed by the spent acid supplied from the third division through I4 and nitric acid from the conduit Il. The spent yacid from the second division flows from the separator 5 through conduit |16 Lto I, being mixed at the same time with the part of the spent acid from the third step supplied through I and with nitric acid from conduit l1. The spent acid from the third step let o" at I3 is diluted with water in the agitator I8, so as to separate the dissolved nitro compounds. After these have been removed.` at centrifuge: |52 the re'- maining diluted acid is added to the spent acid from the first step removed through conduit 2D..

unconverted in the first division` because thisunconverted toluene first of allimpedesthe sep.-A aration of mononitrotoluene and then enters into the second step Where the temperature is higher so that a certain oxidation of toluene. will' take place. On the other hand, it is not desirable to bring about a partialiconversion intodinitrotoluenefin the rst division.

In the second division, the nitration to dinitrotoluene is not carried to` completion. A strong mixed acid is used in the third division, so as to convert the loWer-nitrated product into trinit-rotoluene. Moreover, the meltingpoint of the product in the second division. rises as the amount of dinitrotoluene increases. In any case, the product 0f the second division 'should remain. in liquid state, so that it can more easily be separated and converted. Likewise,. a high melting point of the product would also require a higher temperature and this would` involve anV increased rate of oxidation. conducted in such a Way that, at. the end off the second division of the process on` the average 1.4 to.1.6 nitro grou-ps have been.` introduceddntofeach molecule of toluene.

Under the conditions described,Y inthe second step less nitro groups are introduced nt'o the toluene` than in the rst division. Consequently',4 in the' second division, less nitric acid` will entier into reaction than in the rst division. Therefore, it is recommendable to. keep the; total.I amount of mixed acid in the second. division smaller than intherst division. This caribe accomplished by leading the spent acid' from thel third division partly to the second and partly directly to the rst. Thus, in the rst divisionspent acid: from the second aswell as from the third is obtained;

The composition of ther spent. acidi may be: at the end of the rst division; '71% HSQ4, 25% H2O, 1% HNOa and.3% HNOz; at the end of the 2% HNOz, and 3% nitro-products; andinally, at the end of the third division: 81%. HzSOa. 3"% H2O, 6% HNOa, 3% HNOz and '7% dissolved trinitrotoluene. It will be understood: that these data are only given byl way of example of ciesired values for operation.

In the system described, spent acid is let off in two places, namely part of it at the end of the highest division and the remainder at the end of the first. In the highest,I comprising the removal of asymmetrical trinitroto'luene;. the quantity of removed acid may amounttoi. ai third part of the total amount off spent acid: of: this division. This removed acid is. diluted` with Water; so as to separate the dissolved nitro. com-:- pounds. After these compounds have been; removed, the diluted acid, togetherwith. the spent acid from theffirst division containing only" traces of mononitrotoluenef, may bev leds. to.- the Glover tower of a sulfuric acid plant.

The. operation is preferably 1' sivelyl increasing temperatures;

Mixed acids with at least per cent of free S03 and at least 30 per cent of HNOs are preferably used as starting material. These acids can not be prepared. from nitric and oleic acid, but must be m'ade byV use ofsulfur trioxide (Netherlands Patent Spec. 49,773). As sulfur trioxide has afar stronger capacity to bind water than sulfuric acid has, the nitric acid content of these acids. may be much higher than in the case of sulfuric acid acting as water-binding agent. Remarkably, the oxidizing action of these mixed acids` is, in spiteof their high nitric acid, content not great. An important advantage is furthermore the relatively slight heat evolution during the; nitration with these acids Which facilitates temperature. regulation.

Itis also possible, Within the scope of this invention, to separate the operation of the process into 4* or 5 divisions, thereby subdividing the rst and/or third division of the system just described, butA these variations: are not preferable.

While the exact temperatures used in eachidivision. of the process; Will` depend to some extent upon the concentrations' of themixed acidsyused in the nitration process, We have found. certain temperatures in each division are'preferable for mixedi acids of the proper concentrations as. indicated above in order to. obtain high product yields. Thus, ithas beeny found-z that a temperan ture of` aboutZllo to301 C. should be used in the irst division, a temperature from 40 tof50? C. shoul'dl be used. in thesecond division and a' tem perature of to 85 C.V should. be-used2 in the third' division.

In the. third division of the operation, it is adivantageous to increasev the temperature as. the nitration proceeds, e; g.,.by leading the continu'- ous i'ow of thereaction mixture througha number of spacesv or heat exchangers with progres- Thev tempera.- ture in the third division is' preferably increased Iifi'rogressively from 55'to85f" and can most readily be-accoinplished in three phases of` 55 C., 75 Cl and'85CL, respectively.

The present process results in the production ofl symmetrical trinitrotoluene'. Atl the same time, however, a certain amount of asymmetrical trinitrotoluenes is formed; which must be` separated. from the symmetricalv product. 'I-'hisseparation is effected by having care. thatthe spent acid of the last division contains at mos-17.8% of' water or else at most a few percent of free S03. In an acidmixtureof this composition, the symmetrical' product crystallizes upon cooling', while the asymmetrical product remains di'ssolved, Since reintroduction of! this` spent acid into a lower division` Wouldl result in anaccumulationof asymmetrical products in the process, means areprovided for letting off! a certain part of the spent acid*- of the highest division. In this way an amountof asymmetrical'- productfis removed; which is equalV to the amount which is formed. during the nitration. The crystal mass that isi separated from such an acid by cooling, is centrifuged, then Washed successively with concentrated. sulfuric acid and water, and finally subjected to: asimple treatment. with sulfite, thus yielding aproduct with amelting point of 80.6" C.

Thel present invention` provides a simple, but effective method for the continuous nitrati'on of toluene' to. trinitrotoluene. Because of` the continuous mannerl of operation, it ispossible to carry out. the procedure automatically. Moreover,I simple: and uncomplicated apparatus may tacting toluene in continuous cocurrent flow with mixed acid comprising nitric acid and sulfur trioxide, said contact operation being separated into a plurality of divisions, the nitro product obtained in each division being separated from the ilow of mixed acid, the resulting separated nitro product being then further continuously nitrated by cocurrent contact in a subsequent division with a flow of stronger acid, fresh mixed acid being used in the last division of the process, and the mixed acid for cocurrent flow in each lower division comprising spent acid from a higher division.

2. The process of claim 1, wherein fresh nitric acid is added to the spent acid from the last division to form the mixed acid for use in the loWer divisions of the process.

3. The process of claim 1, wherein the mixed acid in each of said nitration divisions is stronger than the mixed a-cid in the preceding division.

4. A continuous process for the nitration of toluene to trinitrotoluene which comprises nitrating toluene by cocurrent contact with mixed acid comprising nitric acid and sulfur trioxide, said contact operation being separated into three divisions, toluene being nitrated to mononitro- ,toluene in the first division by continuous cocurrent flow of toluene with mixed acid, the resulting mononitrotoluene being nitrated to a mixture of monoand dinitrotoluene in the second division by continuous cocurrent ow of mononitrotoluene and mixed acid, and the resulting mixture of division two being nitrated to trinitrotoluene in the third division by continuous cocurrent flow of said mixture with mixed acid, separating the nitro product obtained in each division from the mixed acid, and introducing the mixed acid from each division into the flow of reactants in the next lower division.

5. The process of claim 3, wherein the spent acid of the third operation division is separated into two portions, one of said portions being introduced into the flow of reactants in the second division and the other of said portions being introduced into the flow of reactants in the first division.

6. A continuous process for the nitration of toluene to trinitnotoluene which comprises nitrating toluene by cocurrent contact with mixed acid comprising nitric acid and sulfur trioxide, said contact operation being separated into three divisions, toluene being nitrated to mononitrotoluene in the rst division by continuous cocurrent ow of toluene with mixed acid at a temperature between 20 to 30 C., the resulting mononitrotoluene being nitrated to a mixture of monoand dinitrotoluene in the second division by continuous cocurrent flow of mononitrotoluene and mixed acid at a temperature of 40 to 50 C., and the resulting mixture of division two being nitrated to trinitrotoluene in the third division Iby continuous cocurrent flow of said mixture with mixed acid at a temperature of 55 to C., separating the nitro product obtained in each divisi-on from the mixed acid, and introducing the mixed acid from each division into the flow of reactants in the next lower division.

7. The :process of claim 4, wherein the temperature of the reactants, while travelling in continuous, cocurrent flow through the apparatus of said division 3, is progressively increased from 55 C. to 85 C.

8. The process of claim 4, wherein the conditions of reaction during the cocurrent flow in the second division is such that said resulting nitro-product mixture has an average of 1.4 to 1.6 nitro groups in each organic molecule.

9. A continuous process for the nitration of toluene to trinitrotoluene which comprises nitrating toluene by cocurrent contact with mixed acid comprising nitric acid and sulfur trioxide in three divisions of reaction mixture floW, toluene being nitrated to mononitrotoluene in the first division by continuous cocurrent ow of toluene with mixed acid at a temperature of between 20 to 30 C., the resulting mononitrotoluene being nitrated to a mixture o f monoand dinitrotoiuene in the second division by continuous cocurrent flow of mononitrotoluene and mixed acid at a temperature of 40 to 50 C., and the resulting mixture of division two being nitrated to trinitrotoluene in the third division by continuous cocurrent ow of said mixture with mixed acid at a temperature of 55 to 85 C., separating the nitro product obtained in each operation division from the mixed acid, and introducing the mixed acid from each division into the flow of reactants in the next lower division, introducing fresh mixed acid having a composition of 30 to 60% S03, 30 to 60% HNOa and 10 to 40% HzSOi into the flow Iof reactants in the third division, proportionlng the mixed acid to organic material in said third division so that the reaction mixture produced by the step has a composition containing less than 2% S03 and less than 8% H2O, cooling said resulting mixture to crystallize symmetrical TNT, and separating said crystallized TNT from the reaction mixture.

THEODDRUSl J. J. HOEK.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,297,170 Holley et al Mar. 11, 1919l 2,012,985 Castner Sept. 3, 1935 2,256,999 Castner Sept. 23, 1941 2,402,180 Papazoni June 18, 1946 FOREIGN PATENTS Number Country Date 48,473 Switzerland Aug .7, 1909 

